With the next generation wireless networks moving towards smaller (micro, pico) cells for providing higher data rates, there is a revived interest in multi-hop wireless networks (MWNs) from the perspective of integrating them with infrastructure wireless networks. With a decrease in cell size, relays are now needed to provide extended coverage, resulting in a multi-hop network.
In a parallel trend, orthogonal frequency division multiple access (OFDMA) has become the popular choice for air interface technology in future infrastructure wireless networks. The entire spectrum is divided into multiple carriers (sub-channels), leading to several physical layer and scheduling benefits. The two-hop network model coupled with OFDMA provides a multitude of diversity and spatial reuse gains such as:                (i) multi-user diversity: for a given sub-channel, different users experience different fading statistics, allowing us to pick a user with a larger gain;        (ii) channel diversity: sub-channels experiencing high gain could vary from one user to another, allowing for multiple users to be assigned their best channels in tandem;        (iii) cooperative diversity: relays can exploit wireless broadcast advantage to cooperate and improve the SNR (signal-noise ratio) at the intended receiver; and        (iv) spatial reuse: simultaneous transmissions on relay hop (BS-RS) and access hop (RS-MS) can be leveraged on the same channel as long as there is no mutual interference. Further, the two-hop nature with infrastructure support makes it possible to effectively leverage these gains, unlike in a large multi-hop setting.        
Relay-enabled wireless networks (e.g. WIMAX 802.16j) coupled with orthogonal frequency division multiple access (OFDMA) as the air interface technology represent an emerging trend in future wireless infrastructure deployments. With the growing popularity of mobile IPTV and location-based services, it becomes essential to jointly optimize future wireless systems for not only unicast but also multicast data. While the OFDM sub-channel structure allows flexible integration of heterogeneous traffic in the system, it also calls for more sophisticated, integrated scheduling solutions that take into account the two-hop nature of the system and efficiently leverage the available diversity and spatial reuse gains.